1. Field of the Invention
This invention broadly relates to a catheter-like device and method for measuring in vivo the velocity of a biological fluid, such as blood. In particular, it relates to a velocity measuring, Doppler crystal, steerable catheter which incorporates an angioplasty, expandable balloon for identifying and treating arterial stenoses.
2. Description of the Prior Art
Coronary artery disease is quite common and is usually manifested in a constriction or stenosis in the arterial tree. An inability to adequately increase flow through stenosed coronary arteries is symptomatic of coronary artery disease. Coronary vasodilator reserve or maximum coronary blood flow is a key indicator of the adequacy of the arterial tree. That is, the coronary vasodilator reserve correlates to the ability of the arterial tree to respond to an increase in myocardial oxygen demand. Hyperemic response or increased blood flow caused by vessel dilation has long been used as a measure of coronary vasodilator reserve. Progressive coronary artery disease can lead to increased vessel stenosis, selective lesions and gradual diminution of the reactive hyperemic response.
Since the ability to adequately analyse changes in blood flow through stenosed coronary arteries is important in properly evaluating the extent of coronary artery disease, many diagnostic tests have been devised to identify the flow limiting characteristics of a particular stenosis. The most common procedure used to predict the physiologic importance of a coronary stenosis is the use of the coronary arteriogram. Such a coronary arteriogram (or angiogram) involves the injection of a radiopaque material (angiodye) into the arterial tree and subsequent radiographic analysis of the extent of the stenosis. Such angiographic analysis is often undertaken concurrent with the inducement of a hyperemic response. Typically, the angiodye induces a certain degree of hyperemic response with other pharmacological agents (e.g. dipyridamole, meglumine diatrizoate, etc.) often used to increase the degree of hyperemia.
Such arteriographic prediction of the effect of coronary arterial disease has recently been criticized for its erratic reliability. For example, interobserver variability error has been shown to be sometimes significant in arteriograph analysis. Further, arteriograph analysis involves a longitudinal cross-sectional view of the vessel in question and usually compares the region in question with the immediately adjacent vessel region. This protocol assumes that the region of the vessel adjacent the lesioned section is normal. Of course, this assumption is often incorrect in that the adjacent region of the vessel may have moderate to severe stenosis which would be readily apparent on a histological or cross-sectional view of the vessel.
In fact, in a recent study (White, et al., Interpretation of the Arteriogram, 310 New Eng. J. Med. 819-824, (1984)) the authors found no significant correlation between the angiographically determined percentage of coronary obstruction and the hyperemic response. Thus, it was concluded that the coronary arteriogram often provides inaccurate information regarding the physiological consequences of the coronary artery disease.
Still other researchers have concluded that the arteriogram is only reliable in identifying a stenosis with greater than about 80% constriction of the vessel. However, marked impairment of the coronary vasodilator reserve can also occur in the 30-80% constriction range, but such stenoses are often not identified by the arteriogram. Thus, while the coronary arteriogram is useful in giving anatomic definition to coronary occlusions, it often provides little information concerning the hemodynamic consequences until near total occlusion of the vessel occurs.
In light of the shortcomings of arteriographic prediction, other methods have been proposed to more accurately analyze coronary stenosis. For example, computer-based quantitative coronary angiographic, coronary video-densitometery and radionuclide-perfusion techniques have all been employed. However, all of the techniques present other difficulties as prediction tools. For example, radionuclide techniques measure regional blood flow, but do not permit continuous assessment of coronary blood flow and are only accurate at low blood flow rates.
In response to the serious drawbacks with current analytical methods employed in assessing the physiological effects of coronary arterial disease, several researchers have experimented with utilizing a catheter which incorporates a Doppler mechanism to measure in vivo the blood velocity (and inferentially blood flow rate). For example, G. Cole and C. Hartley in Pulsed Doppler Coronary Artery Catheter, 56 Circulation 18-25 (1977) proposed a pulse Doppler crystal fitted at the end of a catheter for in vivo analysis. Similarly, Wilson, et al., Diagnostic Methods, 72 Circulation 82-92 (1985) proposed a catheter having a radially-oriented Doppler crystal.
Doppler techniques for measuring flow are advantageous because rapid and dynamic changes in flow can be detected, real-time recordings can be obtained and such techniques are adaptable for miniaturization. In fact, past studies with Doppler catheters appear to have validated the accuracy of such Doppler measurements as an indication of flow. These studies contend that the obstruction of blood flow caused by the catheter is insignificant and that the velocity measurements obtained are linearly related to the actual flow rates. Further, these velocity measurements purportedly accurately track actual flow rates throughout hyperemic response.
To date, however, such Doppler catheters are largely impractical for clinical applications and are beset with technical difficulties. For example, such past Doppler catheters have been of such a size that stenoses located in most parts of the arterial tree cannot be effectively evaluated. Further, such past Doppler-based catheters have not been effectively steerable and thus cannot be accurately placed within the arterial tree. In fact, signal instability and error has been often encountered due to catheter placement and orientation relative to the vessel walls and flow axis. Of course, of primary consideration in a Doppler-based catheter design is the safety to the patient by the electrical isolation of the Doppler crystal.